The erection of above-ground structures often requires the formation of inground retaining walls for use as a load-bearing foundation or as a barrier to prevent the collapse of soil into the excavated area. Where such excavations are made adjacent an existing structure, a retaining wall along the excavation line adjacent the existing structure is necessary to resist soil pressures established beneath the adjoining structure. If no retaining wall is formed, the soil beneath the adjoining structure can collapse outwardly into the excavation and/or damage the existing structure. In addition to the retention of soil, retaining walls of this type are often constructed to block the flow of ground water into the excavated area.
A number of methods have been employed to form retaining walls around an excavation site or adjacent structures which adjoin such site. One method is meant to employ piles formed of wood or steel which are driven along the excavation line to form the retaining wall. Alternatively, a row of bored holes are formed along the excavation line which receive reinforced concrete piles to form the retaining wall. Both of these methods produce retaining walls which are not watertight, and which may require substantial horizontal strengthening to maintain the desired alignment along the excavation line.
Trenching apparatus such as disclosed in U.S. Pat. Nos. 4,681,483 and 4,843,742, both to Camilleri, have been proposed as an alternative to the methods and apparatus of forming retaining walls mentioned above. In trenching apparatus of this type, a supporting base capable of being moved along an excavation or trench line carries an elongated trench digger arm mounted on one side thereof by a boom assembly. The support base is drivingly connected to skids or track assemblies of the type employed in conventional bulldozers or other earth moving equipment which are effective to move the support base and digger arm along the excavation line to form a trench of the desired depth. Concrete is poured into the trench immediately behind the moving trenching apparatus into which appropriate reinforcing bars are inserted so that an essentially continuously formed reinforced concrete retaining wall is provided at the excavation site.
The trenching apparatus described in U.S. Pat. No. 4,681,483 includes a boom assembly having an inner boom which telescopes in and out of an outer boom by operation of linear actuators such as hydraulic or pneumatic extension cylinders. This boom assembly is pivoted between a raised position and a lowered position relative to ground level by another linear actuator, preferably a lift cylinder mounted between the boom assembly and support base. As disclosed in the U.S. Pat. No. 4,681,483, the boom assembly is coupled to the digger arm by a manipulator or work head which provides for pivotal movement of the digger arm about an X axis, a Y axis and a Z axis, all of which are mutually perpendicular. This pivoting structure includes an hydraulic motor carried by the boom assembly having an output shaft connected to the digger arm which is rotatable about the Z axis to pivot the digger arm about such axis. Pivotal motion of the digger arm relative to the Y axis is obtained by operation of a second hydraulic motor, carried by a bracket connected to the boom assembly, whose output shaft is mounted to the digger arm and is rotatable about the Y axis. X axis pivotal motion of the digger arm is produced by operation of a linear actuator having a piston which is extendable and retractable to pivot the digger arm about a pin carried by the boom assembly.
As discussed in detail in the U.S. Pat. No. 4,681,483 patent, the linear actuators associated with the boom assembly, and the pivot mechanisms connected between the boom assembly and digger arm, cooperate to lower the digger arm from ground level to the desired trench depth, and to move the digger arm along the trench line, while continuously maintaining the digger arm in a substantially vertical attitude. Additionally, a platform which carries the boom assembly on the support base is pivotal to allow the boom assembly to swing the digger arm forwardly and rearwardly relative to the support base so that areas close to a structure adjoining the excavation site and/or areas where the trench forms a corner, can be accommodated by such trenching apparatus.
Despite the improvements provided by devices of the type disclosed in the Camilleri U.S. Pat. Nos. 4,681,483 and 4,843,742, problems have been encountered with their construction and method of operation. As mentioned above, two hydraulic motors are employed to rotate or pivot the digger arm with respect to both the Y and Z axes. Because of the substantial weight and length of the digger arm, and the depths of the trenches in at which the digger arm must be operated, it has been found that manipulation of the digger arm by hydraulic motors is not as effective as desired and can result in relatively rapid wear of the shafts, seals and bearings of such motors requiring frequent repair and/or replacement. Additionally, trenching apparatus having a rotatable platform supporting the boom assembly, such as disclosed in the Camilleri patents, adds expense to the overall system in order to obtain the desired swinging movement of the boom assembly.